Purification of fluids involves removal of impurities from fluid steams. Various fluid purification methods are known and practiced. These fluid purification methods generally fall in one of the following categories: absorption into a liquid, adsorption on a solid, permeation through a membrane, chemical conversion to another compound, and condensation. The absorption purification method involves the transfer of a component of a fluid to a liquid absorbent in which said component is soluble. If desired, the liquid containing the transferred component is subsequently stripped to regenerate the liquid. See, for example, A. Kohl and R. Nielsen, "Gas Purification, 5.sup.th edition, Gulf Publishing, 1997; A. Kohl and F. C. Riesenfeld "Gas Purification, 4.sup.th edition, Gulf Publishing, 1985; A. Kohl and F. C. Riesenfeld "Gas Purification, 3rd edition, Gulf Publishing, 1979; and "The Gas Conditioning Fact Book" published by The Dow Chemical of Canada, Limited, 1962; all incorporated herein by reference.
Aqueous solutions of various primary, secondary and tertiary alkanolamines, such as, for example, monoethanolamine(MEA), diethanolamine (DEA), diglycolamine (DGA),diisopropanolamine (DIPA), methyldiethanolamine (MDEA) and triethanolamine (TEA), have been used as absorbent liquids to remove acid gases from liquid and gas streams. In a regeneration method, the aqueous alkanolamine solution containing acid gas is then subjected to heat to regenerate the aqueous alkanolamine solution.
Primary alkanolamines such as MEA and DGA, or secondary alkanolamines such as DEA or DIPA are generally suitable for highly exhaustive removal of CO.sub.2. However they have the disadvantage of requiring a large expenditure of energy for regeneration. Corrosion is also a major concern when using these alkanolamines (especially primary alkanolamines, that is, MEA and DGA) for gas treating applications.
DuPart et al., Hydrocarbon Processing, Parts 1 and 2 March/April 1993, examine the corrosivity of various alkanolamines. They show that the order of corrosivity to carbon steel is MEA&gt;DEA&gt;MDEA.
Tomoe et al., Proceedings of the First Mexican Symposium on Metallic Corrosion, 1994, March 7-11, Merida, Yucatan Mexico, report that after one year of operation with 65 percent by weight DGA the carbon steel and even austinitic stainless steel of the plant was found to be vigorously attacked.
Harruff, L. G., Proceedings of The 1998 Gas Conditioning Conference, Norman, OK, March 1-4, pp. 76-98, also report violent foaming for a plant using DGA. In this particular case, addition of large carbon filter beds in combination with a thermal reclaimer were required to improve operations.
It is also known that aqueous solutions containing about 20 percent by weight MEA or more, due to the corrosivity to carbon steel, often require addition of toxic heavy metals (that is, for example arsenic, antimony or vanadium) to control plant corrosion to acceptable levels.
Another disadvantage of using primary and secondary alkanolamines such as MEA, DEA and DIPA is that CO.sub.2 reacts with these alkanolamines to form degradation compounds such as ureas, oxazolidinones and ethylenediamines.
C. J. Kim, Ind. Eng. Chem. Res. 1988, 27, and references cited therein shows how DEA reacts with CO.sub.2 to form 3-(2-hydroxyethyl)-2-oxazolidinone (HEO), and N,N,N'-tris(2-hydroxyethyl)ethylenediamine (THEED). This reference also shows how DIPA reacts to form 3-(2-hydroxypropyl)-5-methyl-2-oxazolidinone (HPMO). These degradation compounds reduce the amount of alkanolamine available for acid gas pick up, increase the viscosity of the solution, and potentially increase the corrosivity of the solvent.
Tertiary alkanolamines, especially MDEA and TEA, require less energy consumption for regeneration, but since they do not react directly with CO.sub.2, they normally leave from as low as few thousand part per million (ppm) of CO.sub.2 to as much as a few percent CO.sub.2 in the treated fluid stream. Tertiary alkanolamines are, however, suitable for selective removal of H.sub.2 S from a fluid containing both H.sub.2 S and CO.sub.2, since the absorption rate for H.sub.2 S is about the same for all alkanolamines.
It is well known that primary or secondary alkanolamines activators can be used in combination with tertiary alkanolamines to remove CO.sub.2 from fluid streams down to as low as 100 ppm or less requiring less regeneration energy than is required by using the primary or secondary alkanolamines alone.
Dawodu and Meisen, Chem. Eng. Comm., 1996, 144, p. 103, demonstrate, however, that mixtures of MDEA with a primary alkanolamine (MEA) are harder to strip than mixtures of MDEA with secondary alkanolamine (DEA or DIPA).
Holub et al., Proceedings of The 1998 Gas Conditioning Conference, Norman, Okla., March 1-4, pp. 146-160, discloses that MEA corrosion in plants coupled with the higher component vapor pressure of MEA reduces the practicality of using MEA as a formulating agent (see, page 147, paragraph 4). For this reason, up to now, blends of MDEA and secondary alkanolamines are used almost exclusively to increase capacity and reduce corrosion concerns rather than aqueous solutions of primary or secondary alkanolamines alone.
U.S. Pat. Nos. 5,209,914 and 5,366,709 shows how secondary alkanolamine activators such as ethylmonoethanolamine (EMEA) or butylmonoethanolamine (BMEA) can be used with MDEA to afford better CO.sub.2 removal than MDEA alone. However, the aforementioned Holub et al. reference discloses laboratory and plant data showing that secondary alkanolamines methylmonoethanolamine (MMEA) and DEA have very high rates of degradation which leads to corrosion and loss of capacity (see, page 154, paragraphs 1 and 2). The Holub reference further discloses data of MDEA blends formulated with an additive that is neither a primary nor secondary alkanolamine (see, page 151, paragraphs 2 and 3) that reduces the aforementioned disadvantages of formulating blends of primary and secondary alkanolamines and MDEA for gas treating applications. No data on additive solubility or corrosivity was given for comparison.
U.S. Pat. No. 4,336,233 discloses that the use of a combination of piperazine (a secondary amine) and MDEA results in an improved acid gas removal. However, one particular disadvantage of piperazine is that piperazine carbamate formed from the reaction of piperazine and CO.sub.2 is not soluble in the aqueous MDEA/piperazine solution. Thus, the additive level is limited up to about only 0.8 moles/liter, which severely limits the capacity of the solvent, or requires higher circulation rates to treat the same amount of fluid than other MDEA/alkanolamine activator blends require.
Canadian Patent No. 1,091,429 (G. Sartori et al) describes the use of aqueous solutions containing water-soluble primary monoamines having a secondary carbon atom attached to the amino group in gas purification applications. Primary monoamines having a secondary carbon atom attached to the amino group specifically mentioned in this reference as being suitable are 2-amino-1-propanol, 2-amino-1-butanol, 2-amino-3-methyl-1-butanol, 2-amino-1-pentanol, 2-amino-1-hexanol and 2-aminocyclohexanol. However, this reference does not provide degradation, metals solubility (that is, Fe, Ni and Cr solubility) or corrosion data for MEA compared to the primary monoamines having a secondary carbon atom attached to the amino group that might suggest that these primary monoamines are a commercially viable option as a replacement for MEA. This, combined with the high cost of the primary monoamines having a secondary carbon atom attached to the amino group, are the most likely reasons that there are no known gas treating plants using these primary amines solutions as alternatives to MEA. Furthermore, this reference neither teaches nor even suggests that the aqueous blends of the primary monoamines having a secondary carbon atom attached to the amino group such as, for example, 2-amino-1-butanol (2-AB) and MDEA or other tertiary alkanolamines will have unexpectedly low degradation, corrosivity and metals solubility compared to other MDEA blends known in the art.
Chem. Eng. Comm., 1996, Vol. 144, pp. 103-112, "Effects of Composition on the Performance of Alkanolamine blends for Gas Sweetening", describes the influence of blend composition and components on some of the parameters which can be used to monitor the performance of amine blends for aqueous blends of MDEA and MEA, MDEA and DEA, and MDEA and DIPA.
48.sup.th Annual Laurance Reid Gas Conditioning Conference, March 1-4, 1998, pp. 146-160, "Amine Degradation Chemistry in CO.sub.2 Service", describes the degradation chemistry of various ethanolamines in CO.sub.2 service. The paper promotes gas treating solvents which are not formulated with primary or secondary ethanolamines as a solution for the loss rates associated with the use of various ethanolamines such as MDEA, MMEA and DEA.
It is evident that there is still a great need and interest in the gas purification industry for alkanolamine compositions which are aqueous blends of a primary and tertiary alkanolamine which will be effective in the removal of acidic gases from fluid streams and will have low degradation, corrosivity and metals solubility properties compared to alkanolamine blends known in the art.
It has now been discovered that an aqueous mixture comprising a tertiary alkanolamine and a primary alkanolamine having a secondary carbon atom attached to the amino group is not only effective in removing acidic gases from fluid stream but it also has unexpectedly low degradation, corrosivity and metals solubility properties.
In the context of the present invention the term "fluid stream" encompasses both a gaseous stream and liquid stream.